Hybrid electric vehicles (HEVs) are a type of hybrid vehicles and electric vehicles that combine a conventional internal combustion engine (ICE) propulsion system and an electric propulsion system (hybrid vehicle drivetrain). HEVs achieve better fuel economy than conventional vehicles. Several types of HEVs exist, and the degree to which each functions as an electric vehicle varies as well. Hybrid electric cars are the most common of the HEVs; however, there is a recent surge in hybrid electric trucks and buses.
HEVs may be classified based on how power is supplied to their drivetrain. For example, HEVs may be classified as parallel hybrids, series hybrids, or power-split hybrids. Most hybrid vehicles use regenerative braking to recharge the batteries.
HEVs may include a variable frequency drive (VFD), which is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. VFDs may also be known as variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter. Frequency or hertz is directly related to the speed of the motor, more specifically to its rotations per minute (RPM). In some examples, if the vehicle does not need an electric motor to run at full speed, the VFD may be used to ramp down the frequency and voltage to meet the requirements of the load of the HEV. Similarly, as the need for speed requirements changes, the VFD can turn up or turn down the motor speed to meet the speed requirements.
FIG. 1 shows a conventional and common topology of a system 100 of a main power section of a VFD of an HEV. As shown, the VFD includes a three-phase inverter 102 having six MOSFETS M1-M6, each including a diode. The system 100 includes a starter-generator 104 that provides mechanical power to an engine shaft (not shown) during a motor mode and provides electrical power to a battery 106 during a re-generation mode. The system 100 requires bi-directional power flow between the starter-generator 104 and the battery 106. In other words, a current I (e.g., IMotor and IRe-Generation) is able to flow bi-directionally within the DC-Bus 108 based on the operational mode of the vehicle (i.e., motor mode or re-generation mode). The power, i.e., current IMotor, flows from the battery 106 to the starter-generator 104 through the 3-phase inverter 102 during the motor mode, while the power, i.e., current IRe-Generation, flows from the starter-generator 104 (i.e., induction machine) to the battery 106 through the 3-phase inverter 102 during the motor re-generation mode. As can be seen, the 3-phase inverter 102 is configured to operate differently based on the motor mode. For example, during the motor mode, the 3-phase inverter 102 acts as an inverter from DC to 3-phase AC sinusoidal; while the 3-phase inverter 102 acts as a converter from 3-phase AC sinusoidal to DC.
An inductor 110, positioned in series between the first and second capacitors 118, 120, is used due to EMC regulation constraint by the automotive industry to eliminate the emissions from the converter 102 during the re-generating mode. In fact, the inductor 110 is only used during the Re-Generation mode to filter the battery charging current IRe-Generation through the inductor 110 and a first capacitor 118 connected in series with the inductor 110, which together are referred to as an L-C filter 116.
The Energy Stored in the inductor 110 may be calculated as follows:
                              Motor          ⁢                                          ⁢          Mode          ⁢                      :                    ⁢                                          ⁢                      E                          L              ⁢                              -                            ⁢              Motor                                      =                              L            *                          I              Motor              2                                2                                    (        1        )            where EL-Motor is the energy stored in the inductor 110 during motor mode, L is the value of the inductor 110, and IMotor is the current passing through the inductor 110 during motor mode.
                              Re          ⁢                      -                    ⁢          Generation          ⁢                                          ⁢          Mode          ⁢                      :                    ⁢                                          ⁢                      E                          L              ⁢                              -                            ⁢              Re              ⁢                              -                            ⁢              Generation                                      =                              L            *                          I                              Re                ⁢                                  -                                ⁢                Generation                            2                                2                                    (        2        )            where EL-Re-Generation is the energy stored in the inductor 110 during the re-generation mode, and L is the value of the inductor 110, and IRe-Generation is the current flowing from the starter-generator 104 through the 3-phase converter 102 to the battery 106 during the re-generation mode.
The Resonant Oscillation on the L-C filter 116: the current flowing in the inductor 110 is 180° lagging the current flowing in the first and second capacitors 118, 120. As such, the energy stored in the first capacitor 118 and the second capacitor 120 may be calculated as follows:
                                                        E                              C                ⁢                                                                  ⁢                1                                      =                                                            C                  1                                *                                  V                  2                                            2                                ;                ⁢                                  ⁢        and                            (        3        )                                                      E                          C              ⁢                                                          ⁢              2                                =                                                    C                2                            *                              V                2                                      2                          ,                            (        4        )            where EC1 is the energy stored at the first capacitor 118, and EC2 is the energy stored at the second capacitor 120, C1 is the value of the first capacitor 118, and C2 is the value of the second capacitor 120.
The energy exchange between the inductor 110 and the first capacitor 118 as well as the second capacitor 120 with 180° phase differences causes a resonant oscillation in the current during the motor mode and re-generation mode and provides ripple current on both of the capacitor 118 and capacitor 120.
More importantly, the torque of the starter-generator 104 is directly related and proportional to its winding's torque current especially at Field-Oriented-Control scheme of the starter-generator 104, and therefore the Ripple-Current on the DC-Bus 108. In other words, the Resonant-Oscillation current (e.g., IRe-Generation, IMotor) is proportional to the torque applied to the starter-generator 104.
As described previously, the Ripple-Current (Resonant-Oscillation current) on the DC-Bus 108 is proportional to the torque applied to the starter-generator 104, so the performance and the total output power delivered to the starter-generator 104 are greatly limited by the Resonant-Oscillation of the inductor 110 and the first capacitor 118 known together as the L-C filter 116. The Ripple-Current effect may damage the components of the main power and reduce the overall performance of the main power component significantly. The system 100 as described in FIG. 1, reduces the lifetime of the main power component even with a larger power rating of the main power components. In addition, the Ripple-Current effect may increase the emission to the surrounding area from an EMC perspective. As such, the system 100 may not be stable and prevents the starter-generator 104 from being used at its full designed power. Accordingly, there exists a need for a stable system that utilized the full designed power of the starter-generator 104.